1. Field of the Invention
This invention relates to novel heat and fire resistant fibers and is more particularly concerned with a particular class of novel copolyimide fibers and methods for their preparation.
2. Description of the Prior Art
Aromatic polyimide fibers are known and their preparation is taught in the art, see "Man-Made Fibres" by R. W. Moncrieff, Fifth Edition 1970, pgs. 618-619, John Wiley & Sons, Inc., New York, N.Y.; U.S. Pat. No. 3,415,782; and G. B. 1,188,936. Aromatic polyimides are usually found to be organic solvent insoluble. The prior art discloses the preparation of fibers by spinning a solvent soluble intermediate polyamide-acid, or polyamide-acid salt solution either by wet or dry spinning techniques to a polyamide-acid fiber. The resulting fiber is converted to the corresponding polyimide either by heat or chemical methods. Aside from the disadvantage of a second imidizing step, the prior art has an additional limitation. This is the susceptibility of the polyamide-acid polymer to hydrolytic degradation and the concomitant care required in its handling and spinning in order to maintain proper molecular weight and physical properties in the final fiber, see U.S. Pat. No. 3,415,782.
The preparation of soluble aromatic copolyimides is disclosed in U.S. Pat. No. 3,708,458. We have now found that the soluble aromatic copolyimides of the aforesaid U.S. patent can be wet or dry spun directly from their solutions to obtain fibers possessing physical properties comparable to commercial nylon and polyester fibers. The fibers so obtained in the present invention possess good heat and fire resistance. It will be apparent to one skilled in the art that the direct spinning of high temperature resistant, aromatic polyimide fibers represents a marked advance over methods hitherto known and employed in the art.
We have also found that fiber cross-section can be modified by the choice of coagulant fluid employed in the wet spinning process. Fibers are obtained having advantageous properties such as high bulk (low density) good covering properties, good nonconductive (thermal) properties. The fibers can be easily crimped by heat relaxation. When a low molecular weight aliphatic diol, triol, or aqueous solution thereof is used as the coagulant, the fibers are obtained having an irregular cross-section and pseudo-hollow structure which results in the advantageous properties listed hereinabove. Various techniques are known in the fiber art for lowering fiber density by introducing gas into the polymer before spinning. Sodium carbonate incorporated into viscose forms carbon dioxide bubbles when the spinning solution is passed into an acid coagulating bath. Air has been blown intermittently through a single spinneret orifice to introduce air bubbles into viscose (see "Man-Made Fibres", p. 205, supra). The use of the class of coagulants hereinabove described in the present invention achieves the low density fiber directly.